sched: Be more strict about p->is_blocked

Upon entry to try_to_block_task(), p->is_blocked should be false. After all,
the prior wakeup would have made it so per ttwu_do_wakeup().

Ensure this is the case, rather than clearing it in the path that doesn't set
it.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: John Stultz <jstultz@google.com>
Link: https://patch.msgid.link/20260526113322.364017314%40infradead.org

Author: Peter Zijlstra

kernel/sched/core.c

@@ -6676,8 +6676,9 @@ static bool try_to_block_task(struct rq *rq, struct task_struct *p,
 {
 	unsigned long task_state = *task_state_p;
 
+	WARN_ON_ONCE(p->is_blocked);
+
 	if (signal_pending_state(task_state, p)) {
-		p->is_blocked = 0;
 		WRITE_ONCE(p->__state, TASK_RUNNING);
 		*task_state_p = TASK_RUNNING;
 		clear_task_blocked_on(p, NULL);